The present invention relates to a process for producing molten metal (which term includes metal alloys), in particular although by no means exclusively iron, from metalliferous feed material, such as ores, partially reduced ores and metal-containing waste streams, in a metallurgical vessel containing a molten bath.
The present invention relates particularly to a molten metal bath-based direct smelting process for producing molten metal from a metalliferous feed material.
The most widely used process for producing molten iron is based on the use of a blast furnace. Solid material is charged into the top of the furnace and molten iron is tapped from the hearth. The solid material includes iron ore (in sinter, lump or pellet form), coke, and fluxes and forms a permeable burden that moves downwardly. Preheated air, which may be oxygen enriched, is injected into the bottom of the furnace and moves upwardly through the permeable bed and generates carbon monoxide and heat by combustion of coke. The result of these reactions is to produce molten iron and slag.
A process that produces iron by reduction of iron ore below the melting point of the iron produced is generally classified as a xe2x80x9cdirect reduction processxe2x80x9d and the product is referred to as DRI.
The FIOR (Fluid Iron Ore Reduction) process is an example of direct reduction process. The process reduces iron ore fines as the fines are gravity-fed through each reactor in a series of fluid bed reactors. The fines are reduced in solid state by compressed reducing gas that enters the bottom of the lowest reactor in the series and flows counter-current to the downward movement of fines.
Other direct reduction processes include moving shaft furnace-based processes, static shaft furnace-based processes, rotary hearth-based processes, rotary kiln-based processes, and retort-based processes.
The COREX process includes a direct reduction process as one stage. The COREX process produces molten iron directly from coal without the blast furnace requirement of coke. The COREX process includes 2-stage operation in which:
(a) DRI is produced in a shaft furnace from a permeable bed of iron ore (in lump or pellet form) and fluxes; and
(b) the DRI is then charged without cooling into a connected melter gasifier and melted.
Partial combustion of coal in the fluidised bed of the melter gasifier produces reducing gas for the shaft furnace.
Another known group of processes for producing iron is based on cyclone converters in which iron ore is melted by combustion of oxygen and reducing gas in an upper melting cyclone and is smelted in a lower smelter containing a bath of molten iron. The lower smelter generates the reducing gas for the upper melting cyclone.
A process that produces molten metal directly from ores (and partially reduced ores) is generally referred to as a xe2x80x9cdirect smelting processxe2x80x9d.
One known group of direct smelting processes is based on the use of electric furnaces as the major source of energy for the smelting reactions.
Another known direct smelting process, which is generally referred to as the Romelt process, is based on the use of a large volume, highly agitated slag bath as the medium for smelting top-charged metal oxides to metal and for post-combusting gaseous reaction products and transferring the heat as required to continue smelting metal oxides. The Romelt process includes injection of oxygen enriched air or oxygen into the slag via a lower row of tuyeres to provide slag agitation and injection of oxygen into the slag via an upper row of tuyeres to promote post-combustion. In the Romelt process the metal layer is not an important reaction medium.
Another known group of direct smelting processes that are slag-based is generally described as xe2x80x9cdeep slagxe2x80x9d processes. These processes, such as DIOS and AISI processes, are based on forming a deep layer of slag. As with the Romelt process, the metal layer below the slag layer is not an important reaction medium.
Another known direct smelting process which relies on a molten metal layer as a reaction medium, and is generally referred to as the HIsmelt process, is described in International application PCT/AU96/00197 (WO 96/31627) in the name of the applicant.
The HIsmelt process as described in the International application comprises:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a vessel;
(b) injecting into the bath:
(i) a metalliferous feed material, typically metal oxides; and
(ii) a solid carbonaceous material, typically coal, which acts as a reductant of the metal oxides and a source of energy; and
(c) smelting the metalliferous feed material to metal in the metal layer.
The HIsmelt process also comprises post-combusting reaction gases, such as CO and H2, released from the bath in the space above the bath with oxygen-containing gas and transferring the heat generated by the post-combustion to the bath to contribute to the thermal energy required to smelt the metalliferous feed materials.
The HIsmelt process also comprises forming a transition zone above the nominal quiescent surface of the bath in which there are ascending and thereafter descending droplets or splashes or streams of molten metal and slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
A preferred form of the HIsmelt process is characterized by forming the transition zone by injecting carrier gas, metalliferous feed material, solid carbonaceous material and optionally fluxes into the bath through lances that extend downwardly and inwardly through side walls of the vessel so that the carrier gas and the solid material penetrate the metal layer and cause molten material to be projected from the bath.
This form of the HIsmelt process is an improvement over earlier forms of the process which form the transition zone by bottom injection of carrier gas and solid carbonaceous material through tuyeres into the bath which causes droplets, splashes and streams of molten material to be projected from the bath.
The applicant has carried out extensive pilot plant work on the above-described preferred form of the HIsmelt process and has made a series of significant findings in relation to the process.
According to the present invention there is provided a direct smelting process for producing metal from an iron-containing metalliferous feed material in a metallurgical vessel, which process includes the steps of:
(a) forming a molten bath containing molten metal and slag in the vessel;
(b) supplying an iron-containing metalliferous feed material and a solid carbonaceous material to the vessel and smelting metalliferous material to metal in the molten bath, the solids supplying step including injecting at least part of the solid carbonaceous material and a carrier gas into the molten bath via one or more than one downwardly extending lance/tuyere which extends below the location of the nominal surface of the molten bath (as determined under quiescent conditions), whereby the solids and gas injection causes gas flow (xe2x80x9cbath-derived gas flowxe2x80x9d) in and from the molten bath, which gas flow promotes substantial mixing of molten material in the molten bath, which gas flow has a flow rate of at least 0.30 Nm3/s/m2 at the location of the nominal surface of the molten bath (as determined under quiescent conditions), which gas flow entrains molten material in the molten bath and carries the molten material upwardly from the bath as splashes, droplets and streams, whereby at least part of the splashes, droplets and streams of molten material contact the side walls of the vessel and form a protective layer of slag; and
(c) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere and post-combusting reaction gases released from the molten bath, whereby ascending and thereafter descending splashes, droplets and streams of molten material facilitate heat transfer to the molten bath.
The above-described process is one in which there is strong agitation of molten material in the molten bath which results in a thoroughly mixed bath.
The process requires that the injection of solids and gas into the molten bath causes a bath-derived gas flow rate of at least 0.30 Nm3/s/m2 from the surface of the bath. This flow rate ensures that there is sufficient buoyancy uplift of splashes, droplets and streams of molten material from the molten bath to maximise:
(a) heat transfer to the molten bath via subsequently descending splashes, droplets and streams of molten material; and
(b) contact of molten material with the side walls of the vessel which forms a protective layer of slag that reduces heat loss from the vessel.
Item (b) above is a particularly important consideration in the context of the preferred vessel construction of the present invention which includes water cooled panels that form the side walls in the upper barrel section and optionally the roof and water-cooled refractory bricks that form the side walls in the lower barrel section of the vessel.
The flow rate of at least 0.30 Nm3/s/m2 from the surface of the bath is a substantially higher bath-derived gas flow rate than the Romelt process and the deep-slag process such as the DIOS and AISI processes described above and is a significant difference between the process of the present invention and these known direct smelting processes.
By way of particular comparison, U.S. Pat. No. 5,078,785 of Ibaraki et al (assigned to Nippon Steel Corporation) discloses a particular form of a deep-slag process using a rotatable vessel and discloses bottom injection of gas into a metal layer for the purpose of metal bath agitation. The paragraph commencing at line 17 of column 14 discloses that it is preferred that the xe2x80x9cmetal bath agitation forcexe2x80x9d generated by the bottom gas injection be no more than 6 kW/t. The U.S. patent discloses that at higher levels of agitation there may be undesirably high levels of iron dust generation. On the basis of the information provided in the paragraph commencing at line 21 of column 14, a maximum metal bath agitation force of 6 kW/t corresponds to a maximum bath-derived gas flow rate of 0.12 Nm3/s/m2 at the interface between the metal layer and the slag layer. This maximum gas flow rate is considerably below the minimum flow rate of 0.30 Nm3/s/m2 of the present invention.
Preferably the process includes the addition of solid carbonaceous material into the molten bath in an amount selected to maintain a concentration of at least 3 wt % dissolved carbon in metal in the bath. This results in the molten bath being in a strongly reducing condition.
Preferably the bath is in a strongly reducing condition so that the concentration of FeO in slag is less than 8 wt % based on the weight of slag.
Preferably, the process also includes the addition of solid carbonaceous material into the molten bath in an amount selected to be greater than that required for smelting the metalliferous feed material and generating heat to maintain reaction rates so that dust entrained in off-gas leaving the vessel contains at least some excess carbon with the result that there is at least 4 wt % carbon in dust based on the weight of dust.
Preferably the process includes smelting metalliferous material to metal mainly in the metal layer.
Preferably the solids and gas injection in step (b) causes gas flow from the molten bath substantially across the molten bath.
Preferably the gas flow rate is at least 0.35 Nm3/s/m2, more preferably at least 0.50 Nm3/s/m2, from the surface of the molten bath.
Preferably the gas flow rate is less than 0.90 Nm3/s/m2 from the surface of the molten bath.
Typically, slag is a major part and molten metal is the remaining part of the molten material in the splashes, droplets and streams of molten material.
The term xe2x80x9csmeltingxe2x80x9d is understood herein to mean thermal processing wherein chemical reactions that reduce metal oxides take place to produce liquid metal.
It is preferred that the concentration of solid carbon in dust in off-gas from the vessel be in the range of 5 to 90 wt % (more preferably 20 to 50 wt %) of the weight of dust in the off-gas at a rate of dust generation of 10-50 g/Nm3 in the off-gas.
Preferably solids supplying step (b) includes injecting at least part of the metalliferous material and the carbonaceous material and the carrier gas into the molten bath via one or more lance/tuyere.
The injection of metalliferous material and carbonaceous material may be through the same lance/tuyere or separate lances/tuyeres.
Preferably step (c) of the process post-combusts reaction gases, such as carbon monoxide and hydrogen, generated in the molten bath, in a top space above the surface of the molten bath and transfers the heat generated by the post-combustion to the molten bath to maintain the temperature of the molten bathxe2x80x94as is essential in view of endothermic reactions in the molten bath.
Preferably the one or more than one oxygen-containing gas injection lance/tuyere is positioned to inject the oxygen-containing gas into a central region of the vessel.
The oxygen-containing gas may be oxygen, air or oxygen enriched air containing up to 40% oxygen by volume.
Preferably the oxygen-containing gas is air.
More preferably the air is pre-heated.
Typically, the air is preheated to 1200xc2x0 C.
The air may be oxygen enriched.
Preferably step (c) of the process operates at high levels, ie at least 40%, of post-combustion, where post-combustion is defined as:             [              CO        2            ]        +          [                        H          2                ⁢        O            ]                          [                  CO          2                ]            +              [                              H            2                    ⁢          O                ]            +              [        CO        ]            +              H        2              ]  
where:
[CO2]=volume % of CO2 in off-gas;
[H2O]=volume % of H2O in off-gas;
[CO]=volume % of CO in off-gas; and
[H2]=volume % of H2 in off-gas.
Preferably the process operates at 50% or more post-combustion.
It is preferred particularly that the process operates at 60% or more post-combustion.
In some instances a supplementary source of solid or gaseous carbonaceous material (such as coal or natural gas) may be injected into the off-gas from the vessel in order to capture thermal energy in the form of chemical energy.
An example of such supplementary injection of carbonaceous material is injection of natural gas which cracks and reforms, and thus cools, the off-gas whilst enriching its fuel value.
The supplementary carbonaceous material may be added in the upper reaches of the vessel or in the off-gas duct after the off-gas has left the vessel.
Preferably, the one or more than one lance/tuyere extend through the side walls of the vessel and are angled downwardly and inwardly towards the metal layer.
The upward movement of splashes, droplets and streams of molten material from the molten bath forms a molten material-containing zone above the bath. This zone is hereinafter referred to as a xe2x80x9ctransition zonexe2x80x9d.
Preferably the location and operating parameters of the one or more than one lance/tuyere that injects the oxygen-containing gas and the operating parameters that control the transition zone are selected so that:
(a) the oxygen-containing gas is injected towards and penetrates the transition zone;
(b) the transition zone extends upwardly around the lower section of the or each lance/tuyere and thereby shields to some degree the side walls of the vessel from the combustion zone generated at the end of the or each lance/tuyere; and
(c) there is gas continuous space described as a xe2x80x9cfree spacexe2x80x9d which contains practically no metal and slag around the end of the or each lance/tuyere.
Item (c) above is an important feature because it makes it possible for reaction gases in the top space of the vessel to be drawn into the region at the end of the or each lance/tuyere and be post-combusted in the region.
Preferably the process maintains a relatively high (but not too high) slag inventory and uses the amount of slag as a means of controlling the process.
The term xe2x80x9crelatively high slag inventoryxe2x80x9d may be understood in the context of the amount of slag compared to the amount of metal in the vessel.
Preferably, when the process is operating under stable conditions, the weight ratio of metal:slag is between 4:1 and 1:2.
More preferably the weight ratio of metal:slag is between 3:1 and 1:1.
It is preferred particularly that the metal:slag weight ratio be between 2:1 and 1:1.
The term relatively high slag inventory may also be understood in the context of the depth of slag in the vessel.
The amount of slag in the molten bath has a direct impact on the amount of slag that is in the slag-rich transition zone.
The slag is important in the context of minimising heat loss via radiation from the transition zone to the side walls of the vessel.
If the slag inventory is too low there will be increased exposure of metal in the transition zone and therefore increased oxidation of metal and the potential for reduced post-combustion.
If the slag inventory is too high then the one or more than one oxygen-containing gas injection lance/tuyere become buried in the transition zone and this minimises movement of top space reaction gases to the end of the or each lance/tuyere and, as a consequence, reduces potential for post-combustion.
The metalliferous feed material may be any suitable iron containing material. The iron-containing material may be in the form of ores, partially reduced ores, DRI (direct reduced iron), iron carbide, millscale, blast furnace dust, sinter fines, BOF dust or a mixture of such materials.
In the case of partially reduced ores, the degree of pre-reduction may range from relatively low levels (eg to FeO) to relatively high levels (eg 70 to 95% metallisation).
In this connection, the process further includes partially reducing metalliferous ores and thereafter injecting the partially reduced ores into the molten bath.
The metalliferous feed material may be pre-heated.
The carrier gas may be any suitable carrier gas.
It is preferred that the carrier gas be an oxygen-deficient gas.
It is preferred that the carrier gas comprise nitrogen.